The present invention relates to power amplifier circuits and, in particular, to a power amplifier that is capable of delivering to inductive loads large amounts of current at relatively high repetition rates.
A power amplifier that is designed to drive inductive loads typically operates as a high-current switching device. Power amplifiers of this type are used, for example, to drive the yoke mechanism of the horizontal deflection circuitry of a display system employing a cathode ray tube. The switching rate or operating frequency of the power amplifier is one factor that determines the degree of resolution of an image formed by such a display system. A state of the art power amplifier of this type is generally recognized as one whose maximum operating frequency is approximately 64 kHz.
There are several reasons why the maximum operating frequency of such power amplifiers is limited. One reason is the presence of leakage inductance in impedance transformer circuitry that is employed to develop a sufficient amount of current to drive the switching device to which the inductive load is applied. The reactance of the leakage inductance increases as the operating frequency increases and effectively inhibits the production of a drive signal for the switching device. A second reason is that the drive signals produced in conventional drive circuits have current waveforms that do not promote an efficient use of the power required to drive the switching device. Such inefficient use of power creates dissipation problem that make the switching device susceptible to destruction at high switching speeds. A third reason is that the resistance in the inductive load itself causes excessive power dissipation to occur whenever current is delivered through it at high switching rates.
One of the objects of this invention is, therefore, to provide a power amplifier for driving an inductive load with a large amount of current at frequencies in excess of 270 kHz.
Another object of this invention is to provide in such a power amplifier protection for the output signal switching device from destruction resulting from the loss of an input signal or improper power supply voltages.
Another object of this invention is to provide in such a power amplifier filar-wound impedance transformer circuitry that reduces the leakage inductance and thereby extends the operating frequency of the amplifier.
A further object of this invention is to provide in such a power amplifier a transistor switching device that is driven by current waveforms that promote the efficient use of the power generated to operate the device.
Still another object of this invention is to provide in such a power amplifier an impedance transformer circuit that employs a push-pull amplifier subcircuit to produce a high-current waveform with relatively fast transition times.
Yet another object of this invention is to provide such a power amplifier to improve the degree of resolution of an image formed by a display system employing a cathode ray tube.